Reefs Made of Corals Began to Form Massive Structures Again During

What are Coral Reefs

Appearing equally alone forms in the fossil record more than than 400 1000000 years ago, corals are extremely aboriginal animals that evolved into modern reef-edifice forms over the last 25 1000000 years. Coral reefs are unique (eastward.g., the largest structures on earth of biological origin) and complex systems. Rivaling one-time growth forests in longevity of their ecological communities, well-developed reefs reflect thousands of years of history (Turgeon and Asch, in press).

Corals and their Kind

Corals are anthozoans, the largest class of organisms inside the phylum Cnidaria. Comprising over vi,000 known species, anthozoans also include sea fans, bounding main pansies and anemones. Stony corals (scleractinians) make up the largest order of anthozoans, and are the group primarily responsible for laying the foundations of, and building upward, reef structures. For the nearly part, scleractinians are colonial organisms equanimous of hundreds to hundreds of thousands of individuals, called polyps (Barnes, R.D., 1987; Lalli and Parsons, 1995).

Shut-up of Oculina varicosa polyps. (Photo: John Reed)

Every bit members of the phylum Cnidaria, corals have only a express degree of organ evolution. Each polyp consists of three basic tissue layers: an outer epidermis, an inner layer of cells lining the gastrovascular cavity which acts as an internal space for digestion, and a layer called the mesoglea in between (Barnes, R.D., 1987).

Construction of a typical coral polyp. Click here for a detailed cantankerous-sectional view.

All coral polyps share two basic structural features with other members of their phylum. The get-go is a gastrovascular cavity that opens at only one end. At the opening to this cavity, commonly chosen the mouth, food is consumed and some waste product products are expelled. A second feature all corals possess is a circle of tentacles, extensions of the body wall that environment the rima oris. Tentacles help the coral to capture and ingest plankton for nutrient, clear away debris from the mouth, and human activity every bit the beast'south chief means of defense (Barnes, R.D., 1987; Levinton, 1995).

While coral polyps have structurally uncomplicated body plans, they possess several distinctive cellular structures. I of these is called a cnidocyte—a type of cell unique to, and characteristic of, all cnidarians. Found throughout the tentacles and epidermis, cnidocytes contain organelles chosen cnidae, which include nematocysts, a type of stinging cell. Because nematocytes are capable of delivering powerful, often lethal toxins, they are essential to capturing prey, and facilitate coralline agonistic interactions (Barnes, R.D., 1987).

Near corals, like other cnidarians, contain a symbiotic algae chosen zooxanthellae, within their gastrodermal cells. The coral provides the algae with a protected environment and the compounds necessary for photosynthesis. These include carbon dioxide, produced by coral respiration, and inorganic nutrients such as nitrates, and phosphates, which are metabolic waste products of the coral. In return, the algae produce oxygen and assistance the coral to remove wastes. Nearly importantly, they supply the coral with organic products of photosynthesis. These compounds, including glucose, glycerol, and amino acids, are utilized by the coral equally building blocks in the manufacture of proteins, fats, and carbohydrates, equally well as the synthesis of calcium carbonate (CaCO₃). The mutual exchange of algal photosynthates and cnidarian metabolites is the central to the prodigious biological productivity and limestone-secreting capacity of reef building corals (Barnes, R.D., 1987; Barnes, R.S.K. and Hughes, 1999; Lalli and Parsons, 1995; Levinton, 1995; Sumich, 1996).

Salubrious staghorn coral.

Zooxanthellae often are critical elements in the continuing wellness of reef-building corals. As much every bit ninety% of the organic material they industry photosynthetically is transferred to the host coral tissue (Sumich, 1996). If these algal cells are expelled past the polyps, which can occur if the colony undergoes prolonged physiological stress, the host may die shortly afterwards. The symbiotic zooxanthellae also confers its color to the polyp. If the zooxanthellae are expelled, the colony takes on a stark white advent, which is commonly described equally "coral bleaching" (Barnes, R.S.Yard. and Hughes, 1999; Lalli and Parsons, 1995).

(top)

From Polyp to Reef

Massive reef structures are formed when each stony coral polyp secretes a skeleton of CaCO₃. About stony corals accept very small polyps, averaging i to three mm in bore, but entire colonies can grow very large and counterbalance several tons. Although all corals secrete CaCO_three, non all are reef builders. Some corals, such as Fungia sp., are lone and accept single polyps that can grow as large as 25 cm in diameter. Other coral species are incapable of producing sufficient quantities of CaCO_iii to form reefs. Many of these corals exercise not rely on the algal metabolites produced by zooxanthellae, and live in deeper and/or colder waters across the geographic range of most reef systems (Barnes, R.D., 1987; Sumich, 1996).

Two images of the same solitary coral Fungia sp. On the left is a living organism showing its outer covering of living tissue, and clearly visible central mouth. The paradigm on the right shows the underlying skeletal construction of the same organism with the outer covering of living tissue stripped away.

The skeletons of stony corals are secreted by the lower portion of the polyp. This process produces a cup, chosen the calyx, in which the polyp sits. The walls surrounding the loving cup are chosen the theca, and the floor is chosen the basal plate. Thin, calcareous septa (sclerosepta), which provide structural integrity, protection, and an increased surface area for the polyp's soft tissues, extend upwardly from the basal plate and radiate outward from its middle. Periodically, a polyp will lift off its base and secrete a new floor to its cup, forming a new basal plate to a higher place the old one. This creates a minute sleeping accommodation in the skeleton. While the colony is alive, CaCO₃ is deposited, adding partitions and elevating the coral. When polyps are physically stressed, they contract into the calyx so that well-nigh no function is exposed above the skeletal platform. This protects the organism from predators and the elements (Barnes, R.D., 1987; Sumich, 1996).

Major coral reef sites are seen equally reddish dots on this world map. Most of the reefs, with a few exceptions are found in tropical and semitropical waters, between thirty° northward and 30° south latitudes.

At other times, the polyp extends out of the calyx. The timing and extent to which a polyp extends from its protective skeleton often depends on the time of the mean solar day, also as the species of coral. Almost polyps extend themselves furthest when they feed on plankton at nighttime.

In addition to a substantial horizontal component, the polyps of colonial corals are connected laterally to their neighbors past a sparse horizontal sheet of tissue called the coenosarc, which covers the limestone betwixt the calyxes. Together, polyps and coenosarc constitute a thin layer of living tissue over the cake of limestone they have secreted. Thus, the living colony lies entirely above the skeleton (Barnes, R.South.K. and Hughes, 1999).

This large good for you elkhorn coral (Acropora sp.) exhibits an arborescent branching pattern.

Colonies of reef-building (hermatypic) corals exhibit a wide range of shapes, but most tin can be classified within ten general forms. Branching corals have branches that also take (secondary) branches. Digitate corals await similar fingers or clumps of cigars and have no secondary branches. Table corals are table-similar structures of fused branches. Elkhorn coral has big, flattened branches. Foliose corals have broad plate-similar portions ascent to a higher place the substrate. Encrusting corals grow as a thin layer against the substrate. Submassive corals take knobs, columns or wedges protruding from an encrusting base of operations. Massive corals are brawl-shaped or bedrock-like corals which may be small as an egg or large as a firm. Mushroom corals resemble the attached or unattached tops of mushrooms. Cup corals look like egg cups or cups that accept been squashed, elongated or twisted (McManus et al. 1997). While the growth patterns of stony coral colonies are primarily species-specific, a colony's geographic location, environmental factors (e.g., moving ridge action, temperature, calorie-free exposure), and the density of surrounding corals may affect and/or modify the shape of the colony as information technology grows (Barnes, R.D. 1987; Barnes, R.Due south.Yard. and Hughes 1999, Lalli and Parsons, 1995).

In improver to affecting the shape of a colony's growth, environmental factors influence the rates at which various species of corals grow. One of the most significant factors is sunlight. On sunny days, the calcification rates of corals can be twice every bit fast equally on cloudy days (Barnes, R.S.K. and Hughes, 1999). This is likely a part of the symbiotic zooxanthellae algae, which play a unique office in enhancing the corals' ability to synthesize calcium carbonate. Experiments have shown that rates of calcification slow significantly when zooxanthellae are removed from corals, or when corals are kept in shade or darkness (Lalli and Parsons 1995).

Coral core samples reveal horizontal growth lines.

In general, massive corals tend to grow slowly, increasing in size from 0.v cm to 2 cm per year. However, under favorable conditions (high light exposure, consistent temperature, moderate wave action), some species can abound equally much as 4.five cm per year. In contrast to the massive species, branching colonies tend to grow much faster. Under favorable conditions, these colonies can grow vertically past equally much equally 10 cm per year. This fast growth rate is not equally advantageous as it may seem, however. Mechanical constraints limit the maximum size that branching corals tin achieve. As they become larger, a heavier load is placed on the relatively small-scale expanse fastened to the substratum, rendering the colony increasingly unstable. Under these circumstances, the branches are decumbent to snapping off during strong wave action. The opposite is true of the massive-shaped corals, which go more stable every bit they grow larger (Barnes, R.S.K. and Hughes, 1999).

Where Reefs Be

The isle of Moorea (French Polynesia) is in a geographic area that facilitates reef growth. (Photo credit: Dr. Anthony Picciolo)

Reef-building corals are restricted in their geographic distribution. This is because the algal-cnidarian symbiotic machinery needs a narrow and consequent band of ecology conditions to produce the copious quantities of limestone necessary for reef formation. The formation of highly consolidated reefs only occur where the temperature does not autumn below 18°C for extended periods of time. This specific temperature restriction -18°C- does not, notwithstanding, apply to the corals themselves. In Nihon, where this has been studied in detail, approximately half of all coral species occur where the bounding main temperature regularly falls to fourteen°C an approximately 25% occur where it falls to eleven°C (Veron 2000). Many grow optimally in water temperatures between 23° and 29°C, but some tin can tolerate temperatures as high equally 40°C for limited periods of fourth dimension. About require very salty (saline) water ranging from 32 to 42 parts per k. The water must also be clear to permit high low-cal penetration. The corals' requirement for high light too explains why most reef-building species are restricted to the euphotic (lite penetration) zone, approximately seventy m (Lalli and Parsons, 1995).

Generally, there are almost twice as many coral species in Pacific Ocean reefs, such every bit this Fagatele Bay reef, as in Atlantic Sea reefs.

The number of species of corals on a reef declines rapidly in deeper water. High levels of suspended sediments tin can smother coral colonies, clogging their mouths which can impair feeding. Suspended sediments tin also serve to subtract the depth to which low-cal can penetrate. In colder regions, murkier waters, or at depths below seventy m, corals may withal exist on difficult substrates, but their capacity to secrete limestone is greatly reduced (Barnes, R.D., 1987).

In light of such stringent environmental restrictions, reefs generally are confined to tropical and semitropical waters. The diversity of reef corals, i.due east., the number of species, decreases in college latitudes upward to about 30° due north and south, beyond which reef corals are usually not institute. Bermuda, at 32° north latitude, is an exception to this rule because it lies directly in the path of the Gulf Stream'southward warming waters (Barnes, R.D., 1987).

Another factor that seems to affect the variety of reef-building corals is the sea in which they are located. At to the lowest degree 500 reef-building species are known to exist in the waters of the Indo-Pacific region. In comparison, the Atlantic Ocean contains approximately 62 known species. The fossil tape shows that many species once found across the Atlantic, Pacific and Indian Oceans gradually went extinct in the Atlantic, where the affects of ice ages had stiff impacts on the Caribbean area wherein most of the Atlantic reefs reside. Post-obit the closure of the seaway betwixt the Caribbean and the Pacific, several species of corals became restricted to the Caribbean (Veron 2000).

(top)

The Construction of Coral Reefs

Darwin'southward three stages of atoll formation.

Coral reefs begin to form when gratuitous-swimming coral larvae (planulae) adhere to the submerged edges of islands or continents. As the corals grow and expand, reefs accept on ane of three major feature structures—fringing, barrier or atoll.Fringing reefs, which are the most common, project seaward directly from the shore, forming borders along the shoreline and surrounding islands. Bulwark reefs also border shorelines, but at a greater altitude. They are separated from their adjacent land mass by a lagoon of open up, ofttimes deep h2o. If a fringing reef forms around a volcanic island that subsides completely below bounding main level while the coral continues to abound up, an atoll forms. Atolls are usually round or oval, with a central lagoon. Parts of the reef platform may emerge as i or more islands, and breaks in the reef provide access to the fundamental lagoon (Lalli and Parsons, 1995; Levinton, 1995; Sumich, 1996).

In the 1830s, Charles Darwin distinguished between the iii main geomorphological categories of reefs, and suggested that fringing reefs, barrier reefs, and atolls were all related stages in the sequence of atoll reef formation.

All three reef types—fringing, barrier and atoll—share similarities in their biogeographic profiles.Bottom topography, depth, wave and current strength, light, temperature, and suspended sediments all act to create characteristic horizontal and vertical zones of corals, algae and other species. While these zones vary according to the location and type of reef, the major divisions common to well-nigh reefs, as they move seaward from the shore, are the reef flat, reef crest or algal ridge, buttress zone, and seaward slope.

Graphic of typical coral reef zones.

The reef apartment, or back reef, is located on the sheltered side of the reef. It extends outward from the shore; and may be highly variable in character. Varying in width from twenty or 30 meters to more than a few thousand, the reef apartment may range from just a few centimeters to a few meters deep, and large parts may be exposed at low tide. The substrate is formed of coral rock and loose sand. Beds of body of water grasses often develop in the sandy regions, and both encrusting and filamentous algae are common.

Lagoon and back reef (reef flat zone) with exposed coral heads. (Photo: Bully Bulwark Reef Marine Park Dominance)

Because it is so shallow, this area experiences the widest variations in temperature and salinity, merely it is protected from the total force of breaking waves. Reduced water circulation, the accumulation of sediments, and periods of tidal emersions—when the reef is exposed during low tide—combine to limit coral growth. Although living corals may exist scarce except well-nigh the seaward section of this zone, its many microhabitats support the greatest number of species in the reef ecosystem, with mollusks, worms and decapod crustaceans often dominating the visible macrofauna (Barnes, R.D., 1987; Lalli and Parsons, 1995; Sumich, 1996).

Waves break over a reef crest (algal ridge). (Photograph credit: Great Barrier Reef Marine Park Say-so)

The reef crest, or algal ridge, is the highest point of the reef, and is exposed at low tide. Lying on the outer side of the reef, information technology is exposed to the full fury of incoming waves. The width of this zone typically varies from a few, to possibly 50 m. In this severe habitat, a few species of encrusting calcareous red algae flourish, producing new reef material as quickly as the waves erode it. Where moving ridge action is severe, living corals are practically nonexistent, just in situations of more moderate wave action, the reef crest tends to be dominated by stoutly branching corals. These closely growing, robust colonies form ramparts able to withstand the heavy seas. Small venereal, shrimps, cowries and other animals reside in the labyrinthine subsurface cavities of the reef crest, protected from waves and predators (Barnes, R.D., 1987; Lalli and Parsons, 1995; Sumich, 1996).

Aerial photo of spur-and-groove formations in the Florida reef tract.

The outermost seaward gradient (also called the fore-reef) extends from the low-tide mark into deep water. Just below the low-tide mark to approximately 20 m depth is a rugged zone of spurs, or buttresses, radiating out from the reef. Deep channels that slope down the reef confront are interspersed between the buttresses. These alternating spurs and channels may be several meters broad and upwards to 300 k long (Barnes, R.D. 1987; Lalli and Parsons, 1995; Sumich, 1996).

The buttress zone serves ii main purposes in the reef arrangement. First, information technology acts to dissipate the tremendous force of unabating waves and stabilizes the reef structure. 2d, the channels between the buttresses drain debris and sediment off the reef and into deeper water. Massive corals and encrusting coralline algae thrive in this zone of breaking waves, intense sunlight, and arable oxygen. Modest fish inhabit the many holes and crevices on this portion of the reef, and many larger fish including sharks, jacks, barracudas and tunas patrol the buttresses and grooves in search of food (Barnes, R.D., 1987; Lalli and Parsons, 1995; Sumich, 1996).

The dropoff of a reef gradient can extend hundreds of feet downward.

Continuing down the seaward gradient to well-nigh 20 m, optimal lite intensity decreases, merely reduced wave action allows the maximum number of coral species to develop. Outset at approximately 30 to 40 m, sediments accumulate on the gentle gradient, and corals become patchy in distribution. Sponges, bounding main whips, sea fans, and ahermatypic (non-reef-edifice) corals become increasingly abundant and gradually replace hermatypic corals in deeper, darker water (Barnes, R.D., 1987; Lalli and Parsons, 1995; Sumich, 1996).

References

Barnes, R.D. 1987. Invertebrate Zoology; 5th Edition. Fort Worth, TX: Harcourt Caryatid Jovanovich College Publishers. pp. 92-96, 127-134, 149-162.

Barnes, R.Due south.One thousand. and R.Northward. Hughes. 1999. An Introduction to Marine Ecology; third edition. Oxford, Britain: Blackwell Science Ltd. pp. 117-141.

Lalli, C.Thousand. and T.R. Parsons. 1995. Biological Oceanography: An Introduction. Oxford, UK: Butterworth-Heinemann Ltd. pp. 220-233.

Levinton, J.S. 1995. Marine Biology: Function, Biodiversity, Environmental. New York: Oxford Academy Press, Inc. pp. 306-319.

McManus, J.W., Yard.C.A. Ablan, S.One thousand. Vergara, B.M. Vallejo, L.A.B. Menez, K.P.Thou. Reyes, M.L.G. Gorospe and Fifty. Halmarick, 1997. Reefbase Aquanaut Survey Transmission. ICLARM Educational Series. 18, 61p.

Sumich, J.L. 1996. An Introduction to the Biological science of Marine Life, sixth edition. Dubuque, IA: Wm. C. Brown. pp. 255-269.

Turgeon, D.D. and R.K. Asch. In Press. The State of Coral Reef Ecosystems of the United States and Pacific Freely Associated States. Washington D.C.; NOAA.

Veron, JEN. 2000. Corals of the World. Vol 3. Australia: Australian Found of Marine Sciences and CRR Qld Pty Ltd.

greengrecelf.blogspot.com

Source: https://www.coris.noaa.gov/about/what_are/

Share this post